JP7568491B2 - Water-based ink for ink-jet printing - Google Patents

Description

The present invention relates to a water-based ink for inkjet printing and an inkjet recording method.

Inkjet recording is a method of directly ejecting ink droplets from fine nozzles and depositing them on a recording medium to obtain a recorded matter on which characters or images are recorded. This method has become extremely popular due to its many advantages, including the ease and low cost of producing full-color images, the ability to use plain paper as a recording medium, and the fact that the recording medium is not in contact with the ink. In recent years, inks that use pigments as colorants have come to be widely used in terms of the weather resistance and water resistance of recorded matter.
However, unlike dyes, pigment molecules cannot be uniformly dissolved in the ink vehicle, so it is necessary to maintain the pigment's dispersed state, improve storage stability, and improve ejection stability during inkjet recording.
Furthermore, in commercial and industrial printing, pigments are used to record product information on low-liquid-absorbency recording media such as coated paper and resin films, but since a large amount of pigment remains on the surface of the recording medium, the pigment is prone to peeling off, resulting in problems such as low image fastness.

Various proposals have been made to improve the above problems.
For example, Patent Document 1 discloses an inkjet recording method that achieves both image transferability and robustness, which repeats the steps of: applying a reaction liquid containing a viscosity-increasing component onto a transfer body; and applying an ink containing water, a coloring material, and a wax, thereby forming a first image containing a liquid component containing water and a high-boiling-point water-soluble organic solvent and a solid component formed by mixing the reaction liquid and the ink; contacting a porous body with the first image to form a second image from which a portion of the liquid component has been removed; and heating the second image to transfer it onto a recording medium, and the temperature of the first image and the second image are controlled under specific conditions.

JP 2017-144736 A

However, the technique of Patent Document 1 is insufficient in terms of storage stability, ejection stability, and image fastness.
An object of the present invention is to provide a water-based ink for ink-jet printing which is excellent in storage stability and ejection stability and which can give recorded products having excellent image fastness even when recorded on a low-liquid-absorbing recording medium, and an ink-jet recording method which uses the water-based ink.

The present inventors have found that the above problems can be solved by incorporating crosslinked polymer particles, in which a polyolefin wax is incorporated into a specific crosslinked polymer, into the ink.
That is, the present invention provides the following [1] and [2].
[1] A water-based ink for ink-jet printing, comprising a pigment, crosslinked polymer particles containing a polyolefin wax, a water-soluble organic solvent, and water,
A water-based ink for ink-jet printing, comprising a polymer constituting crosslinked polymer particles, the polymer having a structural unit derived from a carboxylic acid monomer and a structural unit derived from a hydrophobic monomer.
[2] An inkjet recording method for recording on a low liquid-absorbent recording medium using the water-based ink according to [1] above.

The present invention provides a water-based ink for inkjet printing that is excellent in storage stability and ejection stability, and can produce recorded material with excellent image fastness even when recorded on a low-absorbency recording medium, and an inkjet recording method that uses the water-based ink.

[Water-based ink for ink-jet printing]
The water-based ink for ink-jet printing of the present invention (hereinafter also referred to as "the ink of the present invention") is a water-based ink for ink-jet printing that contains a pigment, crosslinked polymer particles containing a polyolefin wax, a water-soluble organic solvent, and water, and the polymer that constitutes the crosslinked polymer particles is a polymer having a constituent unit derived from a carboxylic acid monomer and a constituent unit derived from a hydrophobic monomer.

In this specification, the term "water-based" means that water accounts for the largest proportion by mass of the medium in which the pigment is dispersed.
In addition, "recording" is a concept that includes printing and printing out characters and images, and "recorded matter" is a concept that includes printed matter and printed matter on which characters and images are recorded.
The term "low liquid absorption" is a concept that includes low liquid absorption and non-liquid absorption, and means that the amount of water absorbed by the recording medium when the recording medium is in contact with pure water for 100 ms is 0 g/ ^m2 or more and 10 g/m2 ^or less.

The ink of the present invention is excellent in storage stability and ejection stability, and can provide recorded matter having excellent image fastness even when recorded on a low liquid-absorbent recording medium. The reason for this is not clear, but is thought to be as follows.
A method is known in which an organic solvent such as glycol ether having high hydrophobicity is blended into an aqueous ink for a low liquid-absorbent recording medium to enhance fixability to the recording medium. However, when wax particles are blended into the aqueous ink, the organic solvent having high hydrophobicity reduces the dispersion stability and storage stability of the wax particles.
In the ink of the present invention, the wax particles are contained in the aqueous ink in the form of crosslinked polymer particles in which polyolefin wax is contained (encapsulated) in crosslinked polymer particles. The wax particles contained in the crosslinked polymer particles exhibit high storage stability even in the presence of a highly hydrophobic organic solvent, and are considered to improve the ejection stability during inkjet recording.
Furthermore, it is believed that by including a polyolefin wax in the ink of the present invention, the friction resistance of the ink coating formed on the surface of a recording medium is reduced, improving the fastness of the image.

<Pigments>
The pigment used in the present invention may be either an inorganic pigment or an organic pigment.
Examples of inorganic pigments include carbon black and metal oxides, and carbon black is preferred for black inks. Examples of carbon black include furnace black, lamp black, acetylene black, channel black, etc. Examples of white inks include titanium dioxide, zinc oxide, silica, alumina, magnesium oxide, and other metal oxides.
Examples of organic pigments include azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, and quinophthalone pigments.
The hue is not particularly limited, and in the achromatic ink, achromatic pigments such as white, black, and gray can be used, while in the chromatic ink, chromatic pigments such as yellow, magenta, cyan, red, blue, orange, and green can be used.
The above pigments may be used alone or in combination of two or more kinds.

Suitable forms of the pigment used in the ink of the present invention include (i) a pigment that can maintain a dispersed state without a dispersant, i.e., a self-dispersing pigment, (ii) a pigment particle form in which the pigment is dispersed with a surfactant, and (iii) a polymer particle form containing the pigment. Of these, the polymer particle form containing the pigment is preferred from the viewpoints of storage stability, ejection stability, and image fastness.
Here, "polymer particles containing a pigment" (hereinafter also referred to as "pigment-containing polymer particles") refers to particles in which a polymer encapsulates a pigment, particles consisting of a polymer and a pigment, on the surface of which part of the pigment is exposed, particles in which a polymer is adsorbed to part of the pigment, or mixtures thereof. Of these, particles in which a polymer encapsulates a pigment are more preferred.

[Pigment-containing polymer particles]
The polymer constituting the pigment-containing polymer particles (hereinafter, also referred to as "polymer a") is not particularly limited as long as it has at least the ability to disperse the pigment.
Examples of the polymer a include vinyl resins obtained by addition polymerization of vinyl monomers, polyester resins, polyurethane resins, etc. Among these, vinyl resins are preferred from the viewpoints of pigment dispersion stability, storage stability, etc. The polymer a may be appropriately synthesized or may be a commercially available product.
The pigment-containing polymer particles are more preferably crosslinked polymer particles containing a pigment (hereinafter also referred to as "pigment-containing crosslinked polymer particles") obtained by further crosslinking polymer particles containing a pigment with a crosslinking agent.

The polymer a before crosslinking may be either a water-soluble polymer or a water-insoluble polymer, but is preferably a water-insoluble polymer. Even if the polymer used is a water-soluble polymer, the polymer becomes a water-insoluble polymer by subjecting the polymer to a crosslinking treatment.
In this specification, the term "water-insoluble" of a polymer means that when a polymer that has been dried at 105° C. for 2 hours and has reached a constant weight is dissolved in 100 g of water at 25° C., the amount of dissolution is less than 10 g. When the polymer is an anionic polymer, the amount of dissolution is the amount of dissolution when 100% of the anionic groups of the polymer are neutralized with sodium hydroxide.

[Polymer a]
When the polymer a is a vinyl resin, the polymer a preferably contains (a-1) a structural unit derived from an ionic monomer, and more preferably further contains (a-2) a structural unit derived from a hydrophobic monomer and/or (a-3) a structural unit derived from a nonionic monomer.

[(a-1) Ionic Monomer]
As the ionic monomer (a-1), an anionic monomer is preferable. Examples of the anionic monomer include a carboxylic acid monomer and a sulfonic acid monomer, and a carboxylic acid monomer is more preferable.
The carboxylic acid monomer may be at least one selected from (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid, and is preferably (meth)acrylic acid. "(Meth)acrylic acid" means at least one selected from acrylic acid and methacrylic acid.

[(a-2) Hydrophobic Monomer]
The term "hydrophobic" in the (a-2) hydrophobic monomer means that when the monomer is dissolved in 100 g of ion-exchanged water at 25° C. until it is saturated, the amount of the monomer that dissolves is less than 10 g.
Specific examples of the hydrophobic monomer (a-2) include those described in paragraphs [0020] to [0022] of JP-A-2018-83938. Among these, alkyl (meth)acrylates having an alkyl group with 1 to 18 carbon atoms, particularly 1 to 10 carbon atoms, aromatic group-containing monomers having an aromatic group with 6 to 22 carbon atoms, and macromonomers having a polymerizable functional group at one end are preferred, and one or more selected from styrene, α-methylstyrene, and benzyl (meth)acrylate are more preferred.

The macromonomer having a polymerizable functional group at one end is a compound having a number average molecular weight of 500 or more and 100,000 or less, preferably 1,000 or more and 10,000 or less, and the polymerizable functional group can be an acryloyloxy group or a methacryloyloxy group.
As the macromonomer, an aromatic group-containing monomer-based macromonomer is preferable, and examples of the aromatic group-containing monomer constituting the macromonomer include the aromatic group-containing monomers described above.
Specific examples of commercially available styrene-based macromonomers include AS-6(S), AN-6(S), and HS-6(S) manufactured by Toagosei Co., Ltd.

[(a-3) Nonionic Monomer]
The nonionic monomer (a-3) is a monomer that has a high affinity for water or a water-soluble organic solvent, and is, for example, a monomer that contains a hydroxyl group or a polyalkylene glycol chain.
Specific examples of the component (a-3) include those described in paragraph [0018] of JP-A-2018-83938. Among these, one or more selected from methoxypolyethylene glycol (n = 1 to 30) (meth)acrylate and polypropylene glycol (n = 2 to 30) (meth)acrylate are preferred.
The monomer components contained in each of the above components (a-1) to (a-3) can be used alone or in combination of two or more kinds.

(Content of each structural unit in vinyl resin)
The contents of the structural units derived from the components (a-1) to (a-3) in the vinyl resin are as follows, from the viewpoint of improving storage stability, ejection stability, and image fastness.
The content of the (a-1) component is preferably 5 mass% or more, more preferably 10 mass% or more, even more preferably 12 mass% or more, and is preferably 45 mass% or less, more preferably 40 mass% or less, even more preferably 35 mass% or less.
The content of the (a-2) component is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less.

In the case where the component (a-3) is contained, the content thereof is preferably 2 mass% or more, more preferably 5 mass% or more, even more preferably 10 mass% or more, and preferably 45 mass% or less, more preferably 40 mass% or less, even more preferably 35 mass% or less.
The mass ratio of the component (a-1) to the component (a-2) [component (a-1)/component (a-2)] is preferably 0.2 or more, more preferably 0.3 or more, and preferably 1.2 or less, more preferably 0.8 or less, and even more preferably 0.5 or less.

(Production of Polymer a)
The polymer a can be produced by copolymerizing a mixture of the above monomer components (a-1) to (a-3) by a known polymerization method, preferably a solution polymerization method.
The solvent used in the solution polymerization method is not limited, but polar solvents such as water, aliphatic alcohols, ketones, ethers, and esters are preferred, and water, methanol, ethanol, acetone, methyl ethyl ketone, and the like are more preferred.
In the polymerization, a polymerization initiator and a polymerization chain transfer agent can be used. Examples of the polymerization initiator include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble azo polymerization initiators, and examples of the polymerization chain transfer agent include mercaptans.
The polymerization temperature varies depending on the types of polymerization initiator, monomer, and solvent used, but is preferably 30° C. or higher, more preferably 50° C. or higher, and is preferably 95° C. or lower, more preferably 80° C. or lower.
The polymerization atmosphere is preferably a nitrogen gas or inert gas atmosphere.
The vinyl resin is preferably neutralized with a neutralizing agent as described below.

From the viewpoints of storage stability, ejection stability, and image fastness, the number average molecular weight of polymer a is preferably 5,000 or more, more preferably 10,000 or more, and even more preferably 20,000 or more, and is preferably 80,000 or less, more preferably 60,000 or less, and even more preferably 40,000 or less.
The number average molecular weight of the polymer is measured by the method described in the Examples.

[Preparation of pigment-containing polymer particles]
The pigment-containing polymer particles can be efficiently produced as a pigment aqueous dispersion by a method including the following steps 1 and 2.
Step 1: A step of dispersing a pigment mixture containing a pigment, polymer a, an organic solvent, and water to obtain a dispersion. Step 2: A step of removing the organic solvent from the dispersion obtained in step 1 to obtain an aqueous dispersion of pigment-containing polymer particles A (hereinafter, also referred to as "pigment aqueous dispersion (i)"). When polymer a is a vinyl resin, it is preferable to further carry out the following step 3.
Step 3: A step of adding a crosslinking agent to the pigment water dispersion (i) obtained in step 2 to crosslink the pigment-containing polymer particles, thereby obtaining an aqueous dispersion (I) of pigment-containing crosslinked polymer particles (hereinafter, also referred to as “pigment water dispersion (I)”). The pigment-containing polymer particles according to the present invention also include the pigment-containing crosslinked polymer particles obtained in step 3.

(Step 1)
The pigment mixture in step 1 is preferably obtained by a method in which polymer a is dissolved in an organic solvent, and pigment, water, and, if necessary, a neutralizing agent, a surfactant, and the like are added to the obtained organic solvent solution and mixed to obtain an oil-in-water dispersion.
Although there is no limitation on the organic solvent used in step 1, preferred are ketones, ethers, esters, aliphatic alcohols having from 1 to 3 carbon atoms, and from the viewpoint of improving the wettability to the pigment and the adsorption of polymer a to the pigment, more preferred are ketones having from 4 to 8 carbon atoms, and even more preferred is methyl ethyl ketone. When a vinyl resin is synthesized as polymer a by solution polymerization, the solvent used in the polymerization may be used as it is.

When the polymer a has acid groups, it is preferable that at least a part of the acid groups is neutralized with a neutralizing agent, which is believed to increase the charge repulsion force exhibited after neutralization, suppress the aggregation of pigment particles in the water-based ink, and improve the dispersion stability of the pigment.
In the case of neutralization, it is preferable to neutralize so that the pH is 7 or more and 11 or less.
Examples of the neutralizing agent include bases such as sodium hydroxide, potassium hydroxide, ammonia, and various amines, with sodium hydroxide and ammonia being preferred.
The polymer a may be neutralized in advance.
From the viewpoint of improving dispersion stability and storage stability, the amount of the neutralizing agent used is preferably 10 mol % or more, more preferably 15 mol % or more, even more preferably 20 mol % or more, and is preferably 150 mol % or less, more preferably 120 mol % or less, even more preferably 100 mol % or less.
Here, the equivalent amount of the neutralizing agent used can be calculated by the following formula, where polymer a before neutralization is "polymer a'".
Equivalent amount of neutralizing agent used (mol %)=[{weight (g) of neutralizing agent added/equivalent amount of neutralizing agent}/[{acid value of polymer a′ (mg KOH/g)×weight (g) of polymer (B)}/(56×1,000)]]×100

In the dispersion treatment in step 1, the pigment particles can be atomized to a desired particle size only by main dispersion using shear stress. However, from the viewpoint of obtaining a uniform aqueous pigment dispersion, it is preferable to pre-disperse the pigment mixture and then further carry out main dispersion.
As a dispersing machine used for preliminary dispersion, a commonly used mixing and stirring device such as an anchor blade or a dispersing blade can be used.
Examples of means for applying shear stress used in the present dispersion include kneaders such as roll mills and kneaders, high-pressure homogenizers such as microfluidizers, and media-type dispersers such as paint shakers and bead mills. Among these, it is preferable to use high-pressure homogenizers and bead mills from the viewpoint of reducing the particle size of the pigment.
When the dispersion treatment is carried out using a high-pressure homogenizer, the average particle size of the pigment particles in the pigment aqueous dispersion can be adjusted by controlling the treatment pressure and the number of passes.
From the viewpoints of productivity and economy, the treatment pressure is preferably 60 MPa or more and 300 MPa or less, and the number of passes is preferably 3 or more and 30 or less.

(Step 2)
The organic solvent can be removed by a known method in step 2. It is preferable that the organic solvent in the obtained pigment water dispersion (i) has been substantially removed, but it may remain as long as the object of the present invention is not impaired.
In addition, for the purpose of removing coarse particles and the like, it is preferable to further centrifuge the aqueous dispersion from which the organic solvent has been removed, and then filter the liquid layer portion through a filter or the like, and obtain the pigment aqueous dispersion (i) that has passed through the filter or the like.

(Step 3)
In step 3, which is optional, a crosslinking agent is added to the pigment water dispersion (i) obtained in step 2 to crosslink a portion of the carboxy groups of the polymer a constituting the pigment-containing polymer particles, forming a crosslinked structure in the surface layer portion of the pigment-containing polymer particles, thereby obtaining a pigment water dispersion (I) of pigment-containing crosslinked polymer particles. This allows the polymer formed by crosslinking the polymer a to be firmly adsorbed or fixed to the pigment surface, suppressing aggregation of the pigment, and as a result, it is believed that the dispersion stability and storage stability of the obtained ink are further improved.

The crosslinking agent is preferably a polyfunctional epoxy compound having two or more epoxy groups in the molecule, more preferably a compound having two or more glycidyl ether groups, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 4 carbon atoms.
The epoxy equivalent of the crosslinking agent is preferably 90 or more, more preferably 100 or more, and is preferably 300 or less, more preferably 200 or less.
Suitable examples of the crosslinking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

From the viewpoint of improving storage stability, etc., the degree of crosslinking in step 3 is preferably 8 mol % or more, more preferably 10 mol % or more, and is preferably 70 mol % or less, more preferably 50 mol % or less, in terms of the ratio of the molar equivalent number of the crosslinkable functional group of the crosslinking agent to the molar equivalent number of the carboxy group of polymer a.
From the viewpoint of crosslinking reaction efficiency, the temperature of the crosslinking treatment is preferably 40° C. or higher, more preferably 50° C. or higher, and is preferably 90° C. or lower, more preferably 80° C. or lower.

The non-volatile component concentration (solid content concentration) of the resulting pigment water dispersion (I) is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 45% by mass or less, more preferably 40% by mass or less, from the viewpoint of improving the dispersion stability of the pigment water dispersion.
The solids concentration is measured by the method described in the Examples.
From the viewpoint of dispersion stability, the content of the pigment in the pigment water dispersion (I) is preferably 5% by mass or more, more preferably 8% by mass or more, and is preferably 40% by mass or less, more preferably 30% by mass or less.
The mass ratio of the (crosslinked) polymer constituting the pigment-containing (crosslinked) polymer particles in the pigment water dispersion (I) to the pigment [(crosslinked) polymer/pigment] is preferably 0.1 or more, more preferably 0.5 or more, and is preferably 2 or less, more preferably 1.5 or less.
From the viewpoint of dispersion stability, the average particle size of the pigment-containing (crosslinked) polymer particles is preferably 50 nm or more, more preferably 80 nm or more, and is preferably 400 nm or less, more preferably 350 nm or less.
The average particle size is measured by the method described in the Examples.

<Crosslinked polymer particles containing polyolefin wax>
The ink of the present invention contains crosslinked polymer particles containing a polyolefin wax (hereinafter, also referred to as "wax-containing crosslinked polymer particles") from the viewpoint of improving the storage stability and ejection stability of the ink, and from the viewpoint of reducing the frictional resistance of the recording medium surface and improving the fastness of the image.
The polymer constituting the wax-containing crosslinked polymer particles (hereinafter also referred to as "polymer b") has (b-1) a structural unit derived from a carboxylic acid monomer and (b-2) a structural unit derived from a hydrophobic monomer.
The wax-containing crosslinked polymer particles are preferably used in the form of an aqueous dispersion in which the particles are dispersed in an aqueous medium.

[Polyolefin wax]
The polyolefin wax is a wax whose main component is an olefin monomer.
From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the melting point of the polyolefin wax is preferably 100° C. or higher, more preferably 105° C. or higher, and even more preferably 108° C. or higher, and is preferably 140° C. or lower, more preferably 130° C. or lower, and even more preferably 125° C. or lower.

Examples of olefin monomers that are the main component of polyolefin wax include linear olefins and cyclic olefins, and those that are mainly composed of linear olefins having 2 to 6 carbon atoms are preferred, and polyethylene waxes that are mainly composed of ethylene are more preferred. Here, "mainly composed of ethylene" means that the ethylene content is preferably 50% by mass or more, more preferably 65% by mass or more, and even more preferably 80% by mass or more, based on the total components that make up the wax.
Oxidized polyolefin waxes are included in the category of polyolefin waxes, and can be obtained by introducing oxygen atoms or the like into the molecules of high molecular weight polyolefin polymers while adjusting the molecular weight to a desired value by thermal decomposition or the like.
That is, the polyolefin wax is preferably at least one selected from polyethylene wax and oxidized polyethylene wax.

From the same viewpoints as above, the weight average molecular weight of the polyolefin wax is preferably 400 or more, more preferably 600 or more, even more preferably 800 or more, and is preferably 10,000 or less, more preferably 6,000 or less, even more preferably 4,000 or less.
From the same viewpoint as above, the density of the polyolefin wax is preferably 0.890 g/ ^cm3 or more, more preferably 0.900 g/ ^cm3 or more, even more preferably 0.910 g/cm3 or ^more , and preferably 0.985 g/ ^cm3 or less, more preferably 0.980 g/cm3 or ^less , even more preferably 0.975 g/cm3 or ^less .
In the present invention, waxes other than the polyolefin wax may be contained within the range that does not impair the effects of the present invention.
The content of the polyolefin wax in the total amount of wax is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and still more preferably 100% by mass.

The polyolefin wax is preferably used as a dispersion (emulsion) dispersed in an aqueous medium. There is no particular limitation on the method for producing the polyolefin wax, and for example, a method of mixing a polyolefin wax, other waxes that are used as necessary, and a known surfactant to emulsify the mixture. As the surfactant, a nonionic surfactant, an anionic surfactant, etc. can be used.
Suitable examples of commercially available polyethylene waxes include the "Hiwax" series manufactured by Mitsui Chemicals, Inc. and the "Sunwax" series manufactured by Sanyo Chemical Industries, Ltd.

[Polymer b]
The polymer b contains at least (b-1) a structural unit derived from a carboxylic acid monomer and (b-2) a structural unit derived from a hydrophobic monomer.
The polymer b may further contain (b-3) a structural unit derived from a nonionic monomer, and/or (b-4) a structural unit derived from a polyfunctional monomer having two or more polymerizable double bonds.

Specific examples and suitable examples of the (b-1) carboxylic acid monomer are the same as the specific examples and suitable examples of the (a-1) ionic monomer described in the section on polymer A. That is, the (b-1) component is preferably at least one selected from acrylic acid and methacrylic acid.
Specific examples and suitable examples of the hydrophobic monomer (b-2) are the same as those of the hydrophobic monomer (a-2) described in the section on polymer A. That is, the component (b-2) is preferably at least one selected from styrene, α-methylstyrene, and benzyl (meth)acrylate, more preferably styrene.
Specific examples and suitable examples of the nonionic monomer (b-3) are the same as the specific examples and suitable examples of the nonionic monomer (a-3) described in the section of the polymer a.

(b-4) A polyfunctional monomer having two or more polymerizable double bonds is used to make the polymer b into a crosslinked polymer.
From the viewpoint of improving the discharge stability, image fastness, etc., the polymer b needs to be a crosslinked polymer, but when the raw material monomer for producing the polymer b does not contain the component (b-4), a crosslinking treatment using a crosslinking agent must be separately performed in order to make the polymer b a crosslinked polymer. However, when the raw material monomer for producing the polymer b contains the component (b-4), the polymer b can be made a crosslinked polymer during the production of the polymer b.
Specific examples of the component (b-4) include diacrylate compounds such as polyethylene glycol diacrylate, triethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,6-butylene glycol diacrylate, 1,6-hexanediol diacrylate, 1,4-cyclohexanediol diacrylate, neopentyl glycol diacrylate, 1,9-nonanediol diacrylate, polypropylene glycol diacrylate, 2,2'-bis(4-acryloxypropyloxyphenyl)propane, 2,2'-bis(4-acryloxydiethoxyphenyl)propane, and trimethylolpropane triacrylate, dimethacrylate compounds, triacrylate compounds, trimethacrylate compounds, tetraacrylate compounds, hexaacrylate compounds, methylenebisacrylamide, and divinylbenzene. Among these, 1,6-hexanediol diacrylate, 1,4-cyclohexanediol diacrylate, neopentyl glycol diacrylate, and trimethylolpropane triacrylate are preferred from the viewpoint of reactivity.
The polymer b can be produced in the same manner as in the above (production of polymer a).

[Preparation of wax-containing crosslinked polymer particles]
Examples of methods for producing wax-containing crosslinked polymer particles include (i) a method in which a dispersion containing a crosslinked polymer obtained by copolymerizing a monomer mixture containing the above-mentioned components (b-1), (b-2), and if necessary, the component (b-3), and (b-4) a polyfunctional monomer, and a polyolefin wax are subjected to a dispersion treatment; and (ii) a method in which a crosslinking agent is added to a dispersion containing an uncrosslinked polymer obtained by copolymerizing a monomer mixture containing the above-mentioned components (b-1), (b-2), and if necessary, the component (b-3), and a polyolefin wax, and a crosslinking treatment is performed.
The dispersion treatment method and crosslinking treatment method are the same as those in the above-mentioned [Production of Pigment-Containing Polymer Particles], and specific examples and preferred examples are also basically the same.
Suitable examples of the crosslinking agent include one or more selected from polyglycidyl ethers such as 1,6-hexanediol diglycidyl ether, trimethylolpropane polyglycidyl ether, and pentaerythritol polyglycidyl ether.

Among the above manufacturing methods, from the viewpoint of improving the storage stability and ejection stability of the ink, (ii) a method of crosslinking treatment by adding a crosslinking agent to a dispersion containing an uncrosslinked polymer obtained by copolymerizing a monomer mixture containing the above-mentioned components (b-1), (b-2), and, if necessary, (b-3) is preferred. The reason for this is that in the step of dispersing the polyolefin wax, it is considered that the uncrosslinked polymer is more likely to be adsorbed quickly and uniformly on the wax surface than the crosslinked polymer.
The crosslinking reaction of polymer b may be carried out in a system containing or not containing a pigment, but is preferably carried out in a system not containing a pigment, because if the crosslinking reaction is carried out in a system containing both an aqueous pigment dispersion and an aqueous dispersion of wax-containing polymer particles, the balance of electrostatic repulsive forces between dispersed particles in the system may change in the early stage of the crosslinking reaction, which may cause the pigment in the aqueous pigment dispersion to aggregate.

From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the degree of crosslinking of polymer b is preferably 0.4 mol % or more, more preferably 0.6 mol % or more, and even more preferably 0.8 mol % or more, and is preferably 30 mol % or less, more preferably 20 mol % or less, and even more preferably 15 mol % or less.
The degree of crosslinking in the case of crosslinking using the component (b-4) is preferably 0.4 mol% or more, more preferably 0.6 mol% or more, even more preferably 0.8 mol% or more, and is preferably 15 mol% or less, more preferably 10 mol% or less, even more preferably 5 mol% or less.
When crosslinking is performed using a crosslinking agent, the degree of crosslinking is preferably 0.6 mol % or more, more preferably 0.8 mol % or more, and even more preferably 1 mol % or more, and is preferably 30 mol % or less, more preferably 20 mol % or less, and even more preferably 15 mol % or less.
The degree of crosslinking is the equivalent weight of the crosslinking sites in all the monomers constituting the polymer, and is calculated by the following formula:
Degree of crosslinking=[(molar equivalent of monomer forming crosslinking site)×(number of crosslinking sites in monomer backbone forming crosslinking site)/(molar equivalent of all monomers constituting polymer)]×100

From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the acid value of the polymer before crosslinking that constitutes the wax-containing crosslinked polymer particles is preferably 10 mgKOH/g or more, more preferably 20 mgKOH/g or more, even more preferably 30 mgKOH/g or more, and still more preferably 40 mgKOH/g or more, and is preferably 300 mgKOH/g or less, more preferably 250 mgKOH/g or less, and even more preferably 200 mgKOH/g or less.
From the same viewpoints as above, the acid value of the crosslinked polymer constituting the wax-containing crosslinked polymer particles is preferably 10 mgKOH/g or more, more preferably 20 mgKOH/g or more, even more preferably 30 mgKOH/g or more, still more preferably 40 mgKOH/g or more, and is preferably 300 mgKOH/g or less, more preferably 250 mgKOH/g or less, even more preferably 200 mgKOH/g or less.

From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the average particle size of the wax-containing crosslinked polymer particles in the ink of the present invention is preferably 20 nm or more, more preferably 40 nm or more, and even more preferably 60 nm or more, and is preferably 250 nm or less, more preferably 200 nm or less, and even more preferably 150 nm or less.
The average particle size of the polymer particles is measured by the method described in the Examples.

<Water-soluble organic solvent>
The water-soluble organic solvent used in the present invention mainly serves to impart wetting and spreading properties to the recording medium. The water-soluble organic solvent may be liquid or solid at 25° C., but when the organic solvent is dissolved in 100 mL of water at 25° C., the amount of dissolution is 10 mL or more.
From the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink, the boiling point of the water-soluble organic solvent is preferably 110° C. or higher, more preferably 130° C. or higher, and even more preferably 150° C. or higher, and is preferably 250° C. or lower, more preferably 240° C. or lower, and even more preferably 235° C. or lower.
From the same viewpoint as above, the water-soluble organic solvent preferably contains a polyhydric alcohol and a polyhydric alcohol alkyl ether.
Examples of polyhydric alcohols include ethylene glycol, propylene glycol, 1,2-butanediol, 1,2-hexanediol, 1,2-octanediol, 1,8-octanediol, 1,2-decanediol, 1,3-propanediol, 1,4-butanediol, 2-ethyl-1,3-hexanediol, and diethylene glycol.
Among these, one or more selected from propylene glycol, 1,2-butanediol, 1,3-propanediol, 1,4-butanediol, and the like are preferred, with propylene glycol being more preferred.

Examples of polyhydric alcohol alkyl ethers include ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, and tripropylene glycol monomethyl ether.
Among these, at least one selected from dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, and diethylene glycol monobutyl ether is preferred.
The water-soluble organic solvent may further contain organic solvents other than those mentioned above, within the range that does not impair the effects of the present invention.

The ink of the present invention may further contain a surfactant, if necessary, from the viewpoint of improving the wettability to the recording medium.
Examples of the surfactant include nonionic surfactants, silicone-based surfactants, and fluorine-based surfactants, with nonionic surfactants being more preferred.
Examples of nonionic surfactants include polyoxyalkylene alkyl ether surfactants, acetylene glycol surfactants, polyhydric alcohol surfactants, fatty acid alkanolamides, etc. Among these, acetylene glycol surfactants are preferred, and for example, 2,4,7,9-tetramethyl-5-decyne-4,7-diol is more preferred.
Examples of commercially available nonionic surfactants include the Surfynol series manufactured by Nissin Chemical Industry Co., Ltd. and Air Products & Chemicals, and the Acetylenol series manufactured by Kawaken Fine Chemicals Co., Ltd.
The above surfactants may be used alone or in combination of two or more.

(Content of each component in the ink of the present invention)
The content of each component in the ink of the present invention is as follows, from the viewpoint of improving the storage stability, ejection stability, and image fastness of the ink.

(Pigment content)
The content of the pigment in the ink of the present invention is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 2.5% by mass or more, and is preferably 15% by mass or less, more preferably 10% by mass or less, even more preferably 8% by mass or less.
The content of the pigment-containing polymer particles in the ink of the present invention is preferably 2% by mass or more, more preferably 4% by mass or more, even more preferably 6% by mass or more, and is preferably 18% by mass or less, more preferably 16% by mass or less, even more preferably 14% by mass or less, and still more preferably 12% by mass or less.

(Polyolefin wax content)
The content of the polyolefin wax in the ink of the present invention is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, even more preferably 0.8% by mass or more, and preferably 2.5% by mass or less, more preferably 2.2% by mass or less, even more preferably 2.0% by mass or less.
The content of the wax-containing crosslinked polymer particles in the ink of the present invention is preferably 0.8% by mass or more, more preferably 1% by mass or more, even more preferably 1.5% by mass or more, and is preferably 6% by mass or less, more preferably 5% by mass or less, even more preferably 4% by mass or less, and even more preferably 3% by mass or less.

(Content of Water-soluble Organic Solvent)
The content of the water-soluble organic solvent in the ink of the present invention is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, and is preferably 40% by mass or less, more preferably 35% by mass or less, even more preferably 30% by mass or less.

The content of the surfactant in the ink of the present invention is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and is preferably 3% by mass or less, more preferably 2.5% by mass or less, even more preferably 2% by mass or less.
The water content in the ink of the present invention is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less.
The ink of the present invention may contain various additives such as preservatives, pH adjusters, viscosity adjusters, defoamers, rust inhibitors, etc. as optional components depending on the application. In such cases, a portion of the water content may be replaced with the various additives.

(Physical properties of the ink of the present invention)
From the viewpoint of improving storage stability and image density, the viscosity of the ink of the present invention at 32° C. is preferably 2 mPa·s or more, more preferably 3 mPa·s or more, even more preferably 5 mPa·s or more, and is preferably 12 mPa·s or less, more preferably 9 mPa·s or less, even more preferably 7 mPa·s or less. The viscosity of the water-based ink can be measured using an E-type viscometer.
From the viewpoint of improving storage stability and image density, the pH of the ink of the present invention is preferably 7.0 or more, more preferably 7.2 or more, and even more preferably 7.5 or more. From the viewpoint of member resistance and skin irritation, the pH is preferably 11 or less, more preferably 10 or less, and even more preferably 9.5 or less. The pH of the water-based ink can be measured by a conventional method.

[Inkjet recording method]
The ink-jet recording method of the present invention is an ink-jet recording method in which recording is performed on a low liquid-absorbent recording medium using the ink of the present invention.
The ink of the present invention can be loaded into a known ink jet recording device such as a piezoelectric type, and ejected as ink droplets onto a recording medium to record an image or the like.
The ink of the present invention can provide a recorded product having excellent image fastness even in ink jet recording on a low liquid absorbing recording medium.
Examples of low-absorbency recording media include low-absorbency coated paper, art paper, and non-absorbent resin films.
Examples of coated paper include general-purpose glossy paper and multi-color form glossy paper.
Examples of the resin film include transparent synthetic resin films, such as polyester, polyvinyl chloride, polyolefin, and nylon films. These films may be biaxially oriented, uniaxially oriented, or non-oriented films. Among these, polyester films and oriented polypropylene films are preferred, and corona-discharge-treated polyethylene terephthalate (PET) films and corona-discharge-treated biaxially oriented polypropylene (OPP) films are more preferred.

In the following manufacturing examples, preparation examples, working examples, and comparative examples, "parts" and "%" are "parts by mass" and "% by mass" unless otherwise specified. The methods for measuring each physical property are as follows.

(1) Measurement of the number average molecular weight of the polymer The number average molecular weight of the polymer was measured by gel permeation chromatography (GPC apparatus (HLC-8320GPC) manufactured by Tosoh Corporation, columns (TSKgel SuperAWM-H, TSKgel SuperAW3000, TSKgel guardcolum Super AW-H) manufactured by Tosoh Corporation, flow rate: 0.5 mL/min) using a solution in which phosphoric acid and lithium bromide were dissolved in N,N-dimethylformamide to concentrations of 60 mmol/L and 50 mmol/L, respectively, as an eluent, and using a monodisperse polystyrene kit with known molecular weights (PStQuick B (F-550, F-80, F-10, F-1, A-1000), PStQuick C (F-288, F-40, F-4, A-5000, A-500), manufactured by Tosoh Corporation) as a standard substance.
The measurement sample was prepared by mixing 0.1 g of polymer with 10 mL of the eluent in a glass vial, stirring with a magnetic stirrer at 25° C. for 10 hours, and filtering with a syringe filter (DISMIC-13HP, made of PTFE, 0.2 μm, manufactured by Advantec Co., Ltd.).

(2) Measurement of acid value of polymer The resin was dissolved in a titration solvent of toluene and acetone (2:1) in an automatic potentiometric titrator (Kyoto Electronics Manufacturing Co., Ltd., electric burette, model number: APB-610), and titrated with 0.1 N potassium hydroxide/ethanol solution by potentiometric titration, with the inflection point on the titration curve being the end point. The acid value (mg KOH/g) was calculated from the titration amount of the potassium hydroxide solution up to the end point.

(3) Measurement of average particle size of polymer particles in aqueous dispersion The particle size was measured by dynamic light scattering using a laser particle analysis system (manufactured by Otsuka Electronics Co., Ltd., product name: ELS-8000), and calculated by cumulant method analysis.
The measurement conditions were a temperature of 25°C, an angle between the incident light and the detector of 90°, and 100 cumulative measurements, and the refractive index of water (1.333) was input as the refractive index of the dispersion solvent. The measurement sample was prepared by weighing out an aqueous pigment dispersion into a screw tube (No. 5, manufactured by Maruemu Co., Ltd.), adding water so that the solids concentration was 2 x ^10-4 mass%, and stirring with a magnetic stirrer at 25°C for 1 hour.

(4) Measurement of solid content concentration 10.0 g of sodium sulfate, which had been brought to a constant weight in a desiccator, was weighed out into a 30 mL polypropylene container (φ: 40 mm, height: 30 mm), and about 1.0 g of the sample was added thereto and mixed, and then accurately weighed, maintained at 105° C. for 2 hours to remove volatile matter, and further left in the desiccator for 15 minutes, and the mass was measured. The mass of the sample after removing the volatile matter was taken as the solid content, and was divided by the mass of the sample added to obtain the solid content concentration.

(5) Measurement of pH The pH of the ink at 20° C. was measured using a tabletop pH meter (manufactured by Horiba, Ltd., product name: F-71) equipped with a pH electrode (manufactured by Horiba, Ltd., product name: 6337-10D).

Production Example 1 (Production of Water-Insoluble Polymer a)
A monomer mixture was prepared by mixing 8 parts of acrylic acid, 91 parts of styrene, and 1 part of 1,6-hexanediol diacrylate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a polyfunctional monomer. In a reaction vessel, 10 parts of methyl ethyl ketone (MEK), 0.2 parts of 2-mercaptoethanol (polymerization chain transfer agent), and 10% of the monomer mixture were placed and mixed, and the atmosphere was thoroughly replaced with nitrogen gas.
On the other hand, a mixed solution of the remaining 90% of the monomer mixed solution, 0.2 parts of the polymerization chain transfer agent, 30 parts of MEK, and 1.1 parts of an azo-based radical polymerization initiator (2,2'-azobis(2,4-dimethylvaleronitrile, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., product name: V-65) was placed in a dropping funnel, and the monomer mixed solution in the reaction vessel was heated to 65°C while being stirred under a nitrogen atmosphere, and the mixed solution in the dropping funnel was dropped over 3 hours. After 2 hours had elapsed at 65°C from the end of the dropping, a solution in which 0.1 parts of the polymerization initiator was dissolved in 2 parts of MEK was added, and the mixture was further aged at 65°C for 2 hours and at 70°C for 2 hours, and then dried under reduced pressure to obtain a water-insoluble polymer a having a crosslinked structure with 1,6-hexanediol diacrylate (number average molecular weight: 36,000, crosslinking degree: 0.89 mol%, acid value: 62 mgKOH/g).

Production Examples 2 to 6 (Production of Water-Insoluble Polymers b to f)
Water-insoluble polymers b to f were obtained in the same manner as in Production Example 1, except that the amounts of the monomer components in Production Example 1 were changed as shown in Table 1. The results are shown in Table 1.
It should be noted that the water-insoluble polymers d to f do not contain 1,6-hexanediol diacrylate as a monomer component and therefore do not have a crosslinked structure.

Preparation Example I-1 (Preparation of Wax Dispersion A)
10 parts of the water-insoluble polymer a obtained in Production Example 1 was neutralized by adding 2.2 parts of a 5N aqueous sodium hydroxide solution (sodium hydroxide solid content: 16.9%) as sodium ions for neutralizing the carboxyl groups of the polymer a (degree of neutralization: 82 mol %).
Further, 240 parts of ion-exchanged water was added, and 100 parts by mass of polyethylene wax having a melting point of 109 ° C. (HW110P: manufactured by Mitsui Chemicals, Inc., trade name: Hiwax 110P, melting point: 109 ° C., density: 0.920 g / cm ³ , weight average molecular weight: 1000, softening point: 113 ° C.) was dissolved therein by heating at 85 to 95 ° C., and the resulting mixture was dispersed for 30 minutes using an ultrasonic homogenizer while being held at 90 to 95 ° C., and then cooled to room temperature, and a dispersion was obtained by dispersing three times at a pressure of 200 MPa using a microfluidizer (manufactured by Microfluidics, trade name). Ion-exchanged water was added to the resulting dispersion to obtain a dispersion A of crosslinked polymer particles containing wax (solid concentration: 22%, wax: 20%, polymer: 0.2%, average particle size 124 nm, pH 7.4).

Preparation Examples I-2 to I-6 (Preparation of Wax Dispersions BF)
Wax dispersions B to F were prepared in the same manner as in Preparation Example I-1, except that the type of water-insoluble polymer and the amount of the aqueous sodium hydroxide solution were changed as shown in Table 2. The results are shown in Table 2.
In addition, since the wax dispersions D to F use the water-insoluble polymers d to f that do not have a crosslinked structure, the wax-containing polymer particles contained in the wax dispersions D to F do not have a crosslinked structure.

Preparation Example I-7 (Preparation of Wax Dispersion G)
To the wax dispersion D obtained in Preparation Example I-4, 0.4 parts (corresponding to a crosslinking degree of 2.60 mol%) of an epoxy crosslinking agent (1,6-hexanediol diglycidyl ether, manufactured by Nagase ChemteX Corporation, product name: Denacol EX-212, epoxy equivalent 150) and 55.1 parts of ion-exchanged water were added, the mixture was sealed, and heated at 70° C. for 5 hours while stirring with a stirrer to react with the carboxy group of polymer d, thereby obtaining a dispersion G of wax-containing crosslinked polymer particles (wax: 18%, average particle size 113 nm, pH 8.4).

Preparation Example I-8 (Preparation of Wax Dispersion H)
To the wax dispersion E obtained in Preparation Example I-5, 2.5 parts of an epoxy crosslinking agent (product name: Denacol EX-212) (corresponding to a crosslinking degree of 13.2 mol%) and 53.1 parts of ion-exchanged water were added, the container was sealed, and heated at 70°C for 5 hours while stirring with a stirrer to obtain a dispersion H of crosslinked polymer particles containing wax (wax: 18%, average particle size 82 nm, pH 7.8).

Preparation Example I-9 (Preparation of Wax Dispersion I)
A dispersion I of crosslinked polymer particles containing wax (wax: 18%, average particle size 90 nm, pH 7.8) was obtained in the same manner as in Preparation Example I-8, except that in Preparation Example I-8, 2.5 parts of the epoxy crosslinking agent (trade name: Denacol EX-212) was replaced with 6.8 parts (corresponding to a crosslinking degree of 13.2 mol%) of a carbodiimide crosslinking agent (manufactured by Nisshinbo Chemical Inc., trade name: Carbodilite V-10, NCN equivalent 410) and 53.1 parts of ion-exchanged water were replaced with 48.7 parts.

Preparation Example I-10 (Preparation of Wax Dispersion J)
A dispersion J of crosslinked polymer particles containing wax (wax: 18%, average particle size: 105 nm, pH 7.8) was obtained in the same manner as in Preparation Example I-8, except that in Preparation Example I-8, 2.5 parts of the epoxy crosslinking agent (trade name: Denacol EX-212) was replaced with 3.7 parts (corresponding to a crosslinking degree of 13.2 mol%) of an oxazoline crosslinking agent (manufactured by Nippon Shokubai Co., Ltd., trade name: EPOCROS WS-700, oxazoline value: 220) and 53.1 parts of ion-exchanged water were replaced with 51.8 parts.

Preparation Example I-11 (Preparation of Wax Dispersion K)
In Preparation Example I-1, a polyethylene wax having a melting point of 122°C (manufactured by Mitsui Chemicals, Inc., product name: Hiwax 200P, melting point: 122°C, density: 0.970 g/cm ³ , weight average molecular weight: 2000, softening point: 130°C) was used instead of the polyethylene wax having a melting point of 109°C (product name: Hiwax 110P). In the same manner as in Preparation Example I-1, a dispersion K of crosslinked polymer particles containing wax (degree of crosslinking: 0.89 mol%) (wax: 18%, average particle size 109 nm, pH 7.8) was obtained.

<Pigment Water Dispersion>
Preparation Example II-1 (Preparation of Pigment Water Dispersion)
100 parts of the water-insoluble polymer e obtained in Production Example 5 was mixed with 78.6 parts of MEK, and 24.7 parts of a 5N aqueous sodium hydroxide solution (sodium hydroxide solid content: 16.9%) was added as sodium ions for neutralizing the carboxyl groups of the water-insoluble polymer e to neutralize the mixture (degree of neutralization: 25 mol %).
Further, 800 parts of ion-exchanged water was added, and 100 parts of carbon black pigment (C.I. Pigment Black 7, manufactured by Cabot Chemical Corporation, product name: Monarch 800) was added thereto, and the mixture was stirred for 60 minutes using a Disper (manufactured by Asada Iron Works Co., Ltd., product name: Ultra Disper) at 20°C with the Disper blade rotating at 7,000 rpm.
The resulting mixture was subjected to a dispersion treatment using a Microfluidizer (trade name, manufactured by Microfluidics) at a pressure of 200 MPa for 10 passes to obtain a pigment dispersion (solid concentration: 18.1%, pigment: 9%, polymer: 9%).
To the obtained pigment dispersion, 250 parts of ion-exchanged water was added and stirred, and then the MEK was completely removed at 60° C. under reduced pressure. A portion of the water was further removed to adjust the solids concentration to 20% (pigment: 10%), after which 23.8 parts (corresponding to a crosslinking degree of 13.5 mol%) of an epoxy crosslinker (trimethylolpropane polyglycidyl ether, manufactured by Nagase ChemteX Corporation, product name: Denacol EX-321, epoxy value 140) was added, the mixture was sealed, and heated at 70° C. for 5 hours while stirring with a stirrer to obtain a pigment water dispersion of pigment-containing crosslinked polymer particles (solids concentration: 20%, pigment: 10%, polymer: 10%, average particle size: 94 nm).

Example 1 (Preparation of Water-Based Ink)
50 parts of the pigment water dispersion obtained in Preparation Example II-1, 9 parts of the wax dispersion A obtained in Preparation Example I-1 (wax: 20%), 12 parts of propylene glycol, 12 parts of dipropylene glycol monomethyl ether (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), 0.5 parts of a nonionic surfactant (50% solution of 2,4,7,9-tetramethyl-5-decyne-4,7-diol in propylene glycol, manufactured by Nissin Chemical Industry Co., Ltd., product name: Surfynol 104PG50), and 16.5 parts of ion-exchanged water were added (total 100 parts) to obtain a water-based ink 1 (solid concentration: 12%, pigment: 5%, pigment dispersion polymer: 5.0%, wax: 1.8%, wax dispersion polymer: 0.2%).

Examples 2 to 8 and Comparative Examples 1 to 4 (Production of Water-Based Inks)
In the same manner as in Example 1, water-based inks 2 to 8 and 21 to 24 were obtained according to the formulations shown in Table 3.

The storage stability, ejection stability, and image robustness of the resulting water-based ink were evaluated using the methods described below. The results are shown in Table 3.

(1) Evaluation of storage stability Each water-based ink was stored in an environment of 70° C. for 28 days. For each water-based ink before and after storage, the ink viscosity at 32° C. was measured using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd.), and the rate of change in viscosity (%) was calculated using the following formula to evaluate the storage stability.
Viscosity change rate (%)=(ink viscosity after storage/ink viscosity before storage)×100
The lower the rate of change in viscosity, the better the storage stability is judged to be. A value of 105 or less is practically usable.

(2) Evaluation of Discharge Stability Using the water-based ink, an image was printed on A4-sized coated paper (manufactured by Oji Paper Co., Ltd., product name: OK Topcoat+) by the following inkjet recording method.
(Inkjet recording method)
In an environment of temperature 25±1°C and relative humidity 30±5%, a water-based ink was filled into a print evaluation device (manufactured by Tritec Co., Ltd.) equipped with an inkjet recording head (manufactured by Kyocera Corporation, product name: KJ4B-HD06MHG-STDV, piezo type). The head voltage was set to 26V, the driving frequency to 10kHz, the amount of ejected liquid to be ejected to 12pl, the head temperature to 32°C, the resolution to 600dpi, the number of flushings before ejection to 200, and the negative pressure to -4.0kPa. The recording medium was fixed to the conveying table under reduced pressure so that the longitudinal direction of the recording medium was the same as the conveying direction. A print command was transferred to the print evaluation device, and a solid image with a duty of 100% was printed.
After that, the printing evaluation device was stopped for 30 minutes, and the recording head was exposed to the atmosphere. After 30 minutes had passed, the ink was purged from the recording head once, and the state of nozzle chipping when printing was resumed after wiping was observed. The nozzle recovery rate (%) was calculated by the following formula to evaluate the ejection stability.
Nozzle recovery rate (%) = (number of normally ejecting nozzles/total number of nozzles) x 100
The higher the nozzle recovery rate (%), the better the nozzle recovery property is judged to be, and a value of 90% or more is usable for practical use.

(3) Evaluation of image robustness A solid image was printed at a duty of 100% by the inkjet recording method described in (2) above. After that, the printed paper was left to stand for 24 hours, and then a non-printed piece of coated paper cut to 2 x 2 cm was placed on the printed surface using a Gakushin abrasion tester (manufactured by Daiei Scientific Instruments Manufacturing Co., Ltd., product name: RT-300), and the paper was run back and forth 10 times under a load of 10 N, and the density of the transferred ink was measured using a spectrophotometer (manufactured by Sakata Inx Engineering Co., Ltd., product name: Spectroeye).
The lower the density of the transferred ink, the better the image fastness is judged to be, and a value of 0.2 or less is practically usable.

From Table 3, it can be seen that the water-based ink obtained in the examples has superior storage stability and ejection stability compared to the water-based ink obtained in the comparative examples, and can produce printed matter with excellent image robustness.

Claims (9)

A water-based ink for ink-jet printing, comprising a pigment, crosslinked polymer particles containing a polyolefin wax, a water-soluble organic solvent, and water,
the polymer constituting the crosslinked polymer particle is a polymer having a structural unit derived from a carboxylic acid monomer and a structural unit derived from a hydrophobic monomer ,
The water-based ink for ink-jet printing comprises at least one hydrophobic monomer selected from the group consisting of styrene, α-methylstyrene, and benzyl (meth)acrylate . The water-based ink for ink-jet printing according to claim 1, wherein the melting point of the polyolefin wax is 100°C or higher. The water-based ink for ink-jet printing according to claim 1 or 2, wherein the polyolefin wax is at least one selected from polyethylene wax and oxidized polyethylene wax. The water-based ink for ink-jet printing according to any one of claims 1 to 3, wherein the carboxylic acid monomer is at least one selected from acrylic acid and methacrylic acid. The water-based ink for ink-jet printing according to any one of claims 1 to 4, wherein the hydrophobic monomer is styrene. The water-based ink for ink-jet printing according to any one of claims 1 to 5, wherein the polymer constituting the crosslinked polymer particles further has a constituent unit derived from a polyfunctional monomer having two or more polymerizable double bonds. The water-based ink for ink-jet printing according to any one of claims 1 to 6, wherein the water-soluble organic solvent is one or more selected from dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, diethylene glycol monoisopropyl ether, and diethylene glycol monobutyl ether. The water-based ink for ink-jet printing according to any one of claims 1 to 7, wherein the pigment is in the form of polymer particles containing the pigment. An inkjet recording method for recording on a low-liquid-absorbent recording medium using the water-based ink according to any one of claims 1 to 8.